Critical Characteristics and the Irreducible Knee Joint

Opponents of Neo-Darwinian evolution have argued that it
is impossible, because many biological systems require an irreducible
number of parts for the system to have any useful function. The concept
of irreducibility requires a set of characteristics that must exist simultaneously.
Such characteristics are termed critical characteristics. The advantage
of identifying critical characteristics is that they give an indication
of the minimum quantity of design information that must exist simultaneously
in the genetic code for a mechanism to have any useful function. The irreducible
mechanism of the knee joint is shown to contain at least 16 critical characteristics,
each requiring thousands of precise units of information to exist simultaneously
in the genetic code. This demonstrates that the knee could not have evolved
but must have been created as a fully functioning limb joint from the
beginning of its existence.

Introduction

According to the theory of evolution, natural mechanisms
such as limb joints have evolved one characteristic at a time by random
and rare genetic mistakes, called mutations. Evolutionists admit that
mutations generally give rise to disability and suffering, because organisms
are so delicately balanced that random changes tend to cause all kinds
of malfunctions. For example, mutations are known to be responsible for
serious genetic disorders such as hemophilia and cystic fibrosis.1
However, despite the damaging nature of mutations, evolutionists believe
that sometimes there are mutations that produce an improvement in the
functioning of an organism. Evolutionists believe that these ‘favourable
mistakes’ are inherited by offspring and accumulate so that a new
species evolves. Evolution is believed despite a complete absence of indisputable
evidence for the existence of any information-adding mutations.2

Definition of an irreducible mechanism

Evolution is based on a key assumption that natural mechanisms
within organisms can evolve incrementally so that allintermediate
mechanisms have some useful function that gives a survival advantage.
A mechanism that can evolve one characteristic at a time whilst
always having a useful function can be called a reducible mechanism.
A mechanism that cannot evolve one characteristic at a time whilst
always having a useful function can be called an irreducible mechanism.
Behe has recently applied the term irreducible to biochemical systems
that cannot evolve.3 However, opponents of
evolution have used the basic concept of irreducibility for a long time.
For example, the argument that bird flight requires ‘many parts
to be simultaneously present’ has been used for many years.4

Even Darwin himself admitted that evolution
could only produce a reducible mechanism. In his Origin of Species,
Darwin says:

‘If it could be demonstrated that any complex organ existed
which could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down.’ 5

One of the most vocal evolutionists of
our day, Richard Dawkins, is also adamant that evolution can only work
if it is incremental.6

Since an intelligent designer is not restricted to incremental
change, he is able to create irreducible mechanisms without any difficulty.
This means that an intelligent designer has an inherently far greater
capacity for ingenious design than does the process of evolution. It is
interesting to note that evolutionists sometimes admit that the process
of evolution is very restricted compared to intelligent design because
of the constraint of incremental change. For example, Steven Vogel says:

‘… the evolutionary process faces constraints
far more severe than anything impeding human designers. We biologists
recognise these constraints, but we don’t often rise above our
natural chauvinism and make enough noise about them. Every organism
must grow from an initially smaller to an ultimately larger size. Nature
in effect must transmute a motorcycle into an automobile while providing
continuous transportation. The need for growth without loss of function
can impose severe geometrical limitations.’ 7

It is important to
note the confession that biologists do not make enough noise about the
constraints of evolution because there is an obvious contradiction between
the enormous restrictions of evolution and the obvious superiority of
design in the natural world!

Since only intelligent design can produce an irreducible
mechanism, the existence of irreducible mechanisms in nature demonstrates
the existence of an intelligent Designer.8
Identifying irreducible mechanisms in nature is very important, because
evolutionists are very clever at focusing attention on non-essential parts
within mechanisms. For example, Dawkins places much attention on how the
lens in the human eye is not essential to the basic functioning of the
eye and how this supposedly supports the theory of evolution.9

Definition of a critical characteristic

In this paper, the concept of irreducibility is developed
further to the level of an irreducible set of characteristics that must
exist simultaneously for a mechanism to have any useful function. Such
characteristics are termed critical characteristics. Critical characteristics
must not only be simultaneously present, but they must also be simultaneously
and precisely compatible with each other in order to produce the required
physical effects. In general, the critical characteristics of mechanical
mechanisms are often geometrical characteristics, because these must have
precise values whereas other characteristics such as material properties
do not usually have to have precise values. The advantage of identifying
an irreducible set of critical characteristics is that they give an indication
of the minimum quantity of information in the genetic code that must exist
simultaneously and correctly. The identification of a set of critical
characteristics provides overwhelming evidence that a natural mechanism
could not have evolved.

There are basically three types of limb
joint in animals and humans. These are the ball and socket joint (e.g.
hip and shoulder), the pivot joint (e.g. elbow) and the condylar joint
(e.g. knee). The knee joint is the largest and most complex joint in the
human body. The knee is called a condylar joint because of the articulation
between the femur and the tibia, as shown in Figures 1 and 2.10
The femur has two protrusions called condyles. These have a convex curvature
in order to roll and slide against the tibia. The tibia has two concave
grooves that match the condyles of the femur. The two central ligaments
that connect the tibia to the femur are called cruciate ligaments
because of the way they form a cross. These cruciate ligaments fit neatly
inside the space between the two condyles.

The knee joint is an irreducible joint because each of its
four complex parts needs to exist simultaneously and in a complex assembly
to be able to perform its basic function. The two bones are essential
because they perform the rolling and sliding motion. The two cruciate
ligaments are essential because they act as mechanical linkages and perform
a vital guiding function in the joint, as shown in Figure 2. If just one
ligament is removed, then the joint cannot function as a hinge, and the
joint can have no useful function.

Critical characteristics in the knee joint

Figure 2. The irreducible mechanism of
the knee (bones cut away to show cruciate ligaments).

The irreducibility of the knee joint
is most clearly demonstrated by identifying the critical geometrical characteristics
that must be defined in the genetic code. The knee has many critical geometrical
characteristics because the two cruciate ligaments and the two leg bones
form a very sophisticated and precise mechanism, called a four-bar hinge.11

The four-bar hinge mechanism of the knee is shown at various
stages of rotation in Figure 2. These stages of rotation are schematically
presented in Figure 3 to show clearly how the four-bar hinge works. The
cruciate ligaments form the two crossed bars (b & c) whilst the upper
and lower bones effectively form the other two bars (a & d). The cruciate
ligaments are able to pivot where they are attached to the bones (points
1, 2, 3 & 4) because they are made of a non-rigid material. In a four-bar
hinge, the length of each of the four bars remains constant, but the angle
between each bar can change so the lower leg can rotate. One important
feature of the four-bar hinge is that the instantaneous centre of rotation
approximately coincides with the cross-over point of the cruciate ligaments.
This cross-over point moves as the joint opens and closes so that the
knee does not have a fixed point of rotation, as does a simple pivot joint.
The knee joint is a particularly sophisticated kind of four-bar hinge,
because the cruciate ligaments are not rigid and have to be kept taut
by the rolling action of the bones.

Part

Critical characteristics

No. of critical characteristics

Femur bone

Protrusion of two condyles

2

Convex curvature of two condyles

2

Position of ligament attachment points

2

Tibia bone

Concave curvatures of two tracks

2

Position of ligament attachment points

2

Anterior cruciate
ligament

Position of ligament attachment points

2

Length of ligament

1

Posterior cruciate
ligament

Position of ligament attachment points

2

Length of ligament

1

Total

16

Table 1: Critical characteristics in the knee joint.

There are at least 16 critical characteristics in the knee
joint as shown in Table 1. These are geometrical characteristics that are
absolutely essential to the joint. For simplicity, these will be specified
by the order of 16 genes in the genetic code. It could be argued that
the knee joint also requires characteristics to trigger the growth
of the four separate components. However, these have not been included
in Table 1 because the evolutionist may argue that the four components
already happened to exist in some ‘primitive joint’. It could
also be argued that the knee joint requires characteristics to describe
a muscle actuator. However, these too have been left out because the evolutionist
may argue that these already happened to exist in some ‘primitive
joint’. Therefore, these 16 characteristics represent a conservative
estimate of the minimum required critical characteristics in the knee
joint.

If any one of the critical characteristics shown in Table
1 is missing, then the knee cannot function at all. Anyone who has experienced
a ruptured cruciate ligament will know this only too well. The 16 critical
characteristics must not only be present, but must also be precisely compatible
with each other to produce the right physical motion. The two bones must
have a compatible curvature at their interface and this curvature must
also be precisely compatible with the position and movement of the cruciate
ligaments. In particular, the bones must be shaped so as to make the lower
leg rotate around the instantaneous centre of rotation of the four-bar
hinge. If the attachment points are not in the right place on the bones,
then the instantaneous centre of rotation of the knee joint will not be
compatible with the shapes of the bones, and the knee will seize up or
fall apart. The ligaments must also be assembled to the correct attachment
points so that the ligaments form a cross as shown in Figures 2 and 3.
If one of the ligaments was assembled to the wrong attachment point such
that the cross was not formed, then the knee could not function as a hinge
and would be useless.

Figure 3. Schematic diagram of the four-bar
mechanism in the knee joint. See text for explanation.

Since a human characteristic is typically specified by one
gene with about 1,000 chemical units of information, it requires many
thousands of units of information in the genetic code to specify the essential
design information of the four-bar hinge. The theory of evolution proposes
that mutations cause random changes to units of information in the genetic
code and that this leads to evolution. Yet with the knee, many thousands
of precise units of information must be in place simultaneously for the
knee to have any usefulness.

Not only must all critical characteristics be present from
the start but they must also remain unchanged, otherwise this will cause
the physical system to break down. In the case of a healthy knee joint,
if a random change is made to one of the critical characteristics, such
as the position of a ligament, then the knee will cease to function properly
because the position of the ligament is no longer compatible with other
critical characteristics. This shows why random gene mutations generally
cause malfunctions and suffering in living organisms.

The impossibility of an evolved knee

The knee joint presents a major challenge to the evolutionist
because it is unique, and because there are no intermediate forms of joint
between a condylar joint and the other two limb joints found in animals
and humans—the ball and socket joint and the pivot joint. The knee
is widely acknowledged by anatomists as being a completely distinct type
of joint.12 This is because
the knee uses completely different mechanical principles for hinged movement.
Whereas the knee has two mechanical linkages that perform a vital guidance
role (the cruciate ligaments), the joints of the hip, shoulder and elbow
have no such mechanical linkages at all. Whereas the knee rolls and slides,
the joints of the hip, shoulder and elbow only slide. Whereas the knee
has a centre of rotation that moves by up to several centimetres, the
joints of the hip, shoulder and elbow have a fixed centre of rotation.

It is interesting to note that some biology books describe
the knee joint as a ‘highly modified hinge joint’ implying
that the knee must have evolved from the simple pivot joint that exists
in the elbow.10 The use of the term ‘highly modified’
shows that evolutionists are aware that there is a big difference between
a pivot joint and a condylar joint. In fact, the difference is enormous
because there are no known intermediate mechanisms between a simple pivot
hinge and a four-bar mechanism. Indeed, a pivot hinge has none of the
critical characteristics shown in Table 1. In particular, a pivot joint
has nothing remotely like the two crossed cruciate ligaments at the centre
of the joint. The evolutionist might argue that there is a similarity
with the curved surfaces of the bones. However, there is even a distinct
difference in the curvature of the bones between the knee and elbow joints
because of the complex motion of the knee.

It would seem impossible for evolutionism
to explain how an evolutionary process could cause two ligaments to suddenly
become crossed at the centre of a pivot joint, precisely at the same time
that a space is formed to accommodate them, and precisely at the same
time that a complex and compatible rolling motion is formed! The popular
evolutionist Dawkins claims that he is not aware of any type of complex
organ that could not have evolved by ‘numerous successive slight
modifications’.13 However, the knee
joint appears to be one clear example of a mechanism within the human
body that could not possibly have evolved by numerous successive slight
modifications.

Non-essential parts in the knee joint

The anatomy of the knee in Figure 3 is deliberately simplified
in order to identify the parts that are absolutely essential to the most
basic function of the knee. Having demonstrated that there is an irreducible
mechanism within the knee that cannot evolve, it is important to note
that the complete knee contains an extremely efficient and elegant design
with many complex parts. These include a bone at the front of the knee
called the patella (knee-cap) and a fibrous capsule containing several
ligaments, which encloses and supports the joint. There is also a soft
cartilage to reduce shock loads between the bones and an elaborate arrangement
of muscle fibres connected to the front and back of the leg to enable
the movement of the joint to be finely controlled. There is even a lubricating
fluid, called synovial fluid, inside the knee that makes the joint rotate
smoothly and last a long time.

The biomechanics of the knee are also simplified in Figure
3 for clarity. In reality, the ligaments do stretch by a tiny amount when
the knee is in certain positions. There is also a small amount of torsional
freedom between the femur and tibia. These features make the knee joint
an extremely sophisticated mechanism. Indeed, the knee joint is so sophisticated
that human designers have been unable to produce an artificial knee that
has anything approaching the performance of a real knee.

Even though the fine details of the knee joint give wonderful
evidence of design, it is only the irreducibility of the knee mechanism
that can ‘prove’ the theory of evolution to be impossible. Evolutionists
such as Richard Dawkins are experts at focusing attention on non-critical
parts. If evolutionists attempted to discuss the evolution of the knee,
I suspect they would describe in detail how the knee-cap was not actually
essential and how it just appeared and was retained because it gave advantages.
It is most likely that they would also say that the lubricating fluid
was not essential but that it suddenly appeared and remained because it
gave advantages. After giving many such examples, they may lead the reader
into thinking that every single part of the knee could just evolve by
chance. It is therefore very important to present to evolutionists the
irreducible mechanism at the core of the knee joint and to identify critical
characteristics.

Non-critical characteristics in the knee joint

Non-critical characteristics are characteristics
that do not have to be simultaneously present with exact values for a
system to perform a useful function. Examples of non-critical characteristics
in the knee joint include the colours and material properties of the cruciate
ligaments and bones. Since these characteristics do not interact precisely
with other characteristics and they do not affect the basic functioning
of the knee, it is theoretically possible for these non-critical characteristics
to evolve by mutation and in isolation. However, even though the ligaments
and bones could in theory change in terms of colour and material properties
by random mutations, these changes would not convert the knee into another
kind of mechanism!

The fact that evolution can in theory evolve characteristics
that are non-critical is used by the evolutionist to give the impression
that evolution can work. School textbooks often give examples of how a
new colour of a creature such as a moth could evolve by mutation, and
then say that with many mutations the moth could have evolved from a primitive
creature. However, even though the colour of a moth may be important to
its survival, the characteristic of colour is nevertheless a trivial one
in terms of how it affects the functioning of organs and parts
within the moth. Therefore, the example of the evolution of colour by
mutation is not an example of evolution at all because no matter how many
non-critical characteristics are evolved, they can never change one kind
of functioning system into another kind of functioning system.

When discussing evolution, biology books will rarely distinguish
between critical and non-critical characteristics in an organism although
characteristics are known to vary greatly in importance. This is very
surprising, because a characteristic like the attachment position of a
ligament is vastly more important than the colour of the ligament. There
is no doubt that critical characteristics are obscured because evolutionists
can only attempt to give trivial examples of evolution such as changes
in colour. To demonstrate the theory of evolution, the evolutionist would
have to show how a critical characteristic like the attachment position
of the cruciate ligaments could evolve. However, this has never been done
and can never be done because such a critical characteristic could not
evolve in isolation.

Four-bar hinges in engineering

Four-bar hinges are commonly found in vehicle steering mechanisms.
Engineers always start the design exercise with a complete set of parts.
Following on from the transport analogy of Steven Vogel, the theory of
evolution is analogous to proposing that one can take the engineering
drawings of a simple pivot joint used in a motorbike steering wheel and
evolve them into the drawings of the steering system of a four-wheeled
vehicle. The information on the drawings is equivalent to the genetic
code, and random photocopying errors in the information are analogous
to mutations. The evolutionist believes that the random photocopying errors
will sometimes produce a slightly better system, and that via selection,
eventually the steering system of the motorbike will turn into a four-bar
hinge and form the steering system of a four-wheeled vehicle!

Such reasoning is absurd for several reasons. Firstly, if
a random change were made to the information on a drawing of a motorbike
steering system, then this would at best cause no change in the basic
functions and at worst have catastrophic consequences. Secondly, there
are no intermediate mechanisms between a motorbike steering system and
a car steering system, whereas evolution would require hundreds of fully
functioning intermediate forms. In a similar way, it is impossible for
the knee joint to have evolved from a simple pivot joint such as the elbow
joint.

The uniqueness of the human knee

The basic principle of the knee joint is unique whether
it is the knee joint of an animal or human being. However, there is yet
a further problem for the evolutionist in that the human knee is distinctly
different from animal knees. In the case of humans, the knee is designed
to lock easily in extension (straight leg) so that maintaining straight
legs and a vertical posture is easy. This design feature is one reason
why man is a biped (two-legged) and is able to walk and run upright in
a completely natural way. Apes’ knees cannot lock and must be continually
loaded in flexion (bent leg). Thus apes are generally quadrupedal (four-legged)
and it is extremely difficult for apes to maintain a vertical posture
with its legs. Evolutionists admit that the only way apes can attempt
to stand upright is by having awkward bends at the ankle, knee and hip
joints.14 Such a distorted posture means
that apes can only stay vertical for short periods and distances. In contrast,
an able-bodied and fit human being can run many miles without great difficulty!

Evolutionists admit that there is a key difference between
the knees of animals and humans. For example, Dye says:

‘Despite the overall similarity of the design of
the knee in tetrapods, no ideal animal model of the human knee is available.’
15

That there are thousands of different types of animals on
the earth that move with a horizontal stature provides great evidence
that humans have been deliberately designed to be unique.

The power and wisdom of God

The human knee joint not only gives evidence of design but
it also gives evidence of the infinite power and wisdom of God. Solomon
spoke of the wonder of growth in the womb:

‘As you do not know what is the way of the wind, or
how the bones grow in the womb of her who is with child, so you do not
know the works of God who makes everything’ (Ecclesiastes 11:5).

Conclusion

Whether gene mutations are random (as atheists believe)
or planned (as many theistic evolutionists believe) the process of evolution
cannot produce an irreducible mechanism because evolution is restricted
to incremental change in the genetic code.

The human knee joint is an irreducible mechanism that must
have at least four complex parts existing simultaneously and in a complex
assembly to perform any useful function. The 16 critical characteristics
in the knee joint correspond to several thousand units of information
in the genetic code. These units of information cannot evolve incrementally
but must exist simultaneously for the knee to perform its basic function.

There are no intermediate forms of joint between the condylar
joint of the knee and the other two joints found in animals and humans—the
ball and socket joint and the pivot joint. And there are distinct differences
between the knee joint of animals and that of humans.

There is thus overwhelming evidence that the knee was created
as a fully functioning limb joint from the beginning of its existence.

Stuart Burgess is a lecturer
in Engineering Design in the Department of Mechanical Engineering at Bristol
University, United Kingdom. He has also lectured Engineering Design at
Cambridge University where he was a Bye-Fellow of Selwyn College. He has
published over 50 papers and patents in the area of engineering design
and is a recipient of the Worshipful Company of Turners Engineering Design
Gold Medal. He is a member of Buckingham Chapel in Bristol and has a Diploma
in Theology from the London Reformed Baptist Seminary. He acknowledges
helpful comments from Dr John Davis from the Department of Civil Engineering,
Bristol University. Return to text.

Burgess, S.C., Hallmarks of Design, Day One
Publications, to be published January 2000. This book examines several
irreducible mechanisms in nature and also examines several other ‘hallmarks
of design’. Return to text.